The vast majority of all organic polymeric materials undergo some mode of degradation when exposed to the high energy photons of ultraviolet radiation. The degradation manifests itself depending on the polymeric material in yellowing, discoloration, embrittlement and other loss of physical properties. Polycarbonate resin is no exception and it is, therefore, an object of this invention to provide a method of producing a polycarbonate resin article which is highly resistant to ultraviolet radiation degradation.
The use of ultraviolet radiation absorbers with various resins, such as polyesters, polyolefins, vinyls, and polystyrene to provide protection against attack by ultraviolet radiation is known in the art. The ultraviolet radiation absorber functions by reason of its ability to screen out the damaging ultraviolet portion of light due to its very high absorptivity relative to that of the polymer. In order to qualify as a successful ultraviolet light absorber for a polymer, particularly for polycarbonate, there are several requirements which the absorber must fulfill. The absorber must have a high specific absorptivity in the range of wave lengths that are most deleterious to the polymer and that are present in the source of the exposure. The absorber must be compatible with the polymer such as polycarbonate and must not degrade the polymer with loss of properties and increase in color. The absorber must not significantly absorb in the visible region of the spectrum or a color will be imparted to the polymer to which it has been added. The absorber must also have a sufficiently low volatility to permit its continued residence in the polymer.
Several methods are known in the prior art utilizing these ultraviolet radiation absorbers to stabilize various polymers, including polycarbonate, against ultraviolet radiation. These methods include blending the ultraviolet radiation absorbers with the polymer prior to processing; incorporating the absorbers in surface laminating or coating materials which are applied onto the surface of the processed polymer; and impregnating the absorbers in the polymer surface. The surface impregnation techniques known in the prior art include (i) using aggressive solvents to swell or soften the polymer surface thereby allowing the absorber to diffuse into the softened surface of the polymer; (ii) melting the absorber and the polymer surface in order to diffuse the molten absorber into the molten polymer surface; and (iii) partitioning of the absorber between a polymeric surface and a relatively poor solvent for the absorber held at high temperatures whereby the absorber, which is more soluble in the polymer than in the solvent, diffuses into the polymer surface.
While each of these methods can be utilized to impart improved ultraviolet stability to a polymer system, each of them has certain disadvantages. Blending the absorber with the bulk polymer results in the absorber being distributed throughout the entire polymer system. This procedure is both uneconomical, as these absorbers are usually quite expensive, and not completely successful. Since most of the absorber resides in the polymer's interior instead of at the surface where it is most needed, much of the harmful ultraviolet radiation penetrates and deteriorates the surface of the polymer structure before reaching the majority of the interiorly distributed absorber. Furthermore, since the concentration of the absorber in the resin is limited by the degree of compatibility of the absorber with the polymer, using sufficiently high concentrations of absorber effective to provide surface protection generally tends to adversely affect the physical properties of the polymer. Incorporating the absorbers in surface laminating or coating materials suffers from the disadvantage of being difficult and expensive to use since an extra complicated processing step is required. Furthermore, difficulties are sometimes encountered in adhering the coating or laminating material to the surface of the polymer, or in maintaining continued adequate adhesion, especially after exposure to weathering. Even when the coating or laminating material adheres well, they often cannot be applied without forming unsightly streaks on the polymer surface. An additional drawback to this method is that often the physical properties of the polymer, such as impact strength, are adversely affected by these coating or laminating materials.
While, in principle, the surface impregnation techniques are the most desirable since the ultraviolet radiation absorbers are contained only in the surface regions of the polymer where they are needed, in practice the prior art surface impregnation techniques all suffer from certain disadvantages. Melting the polymer and the absorbers in order to diffuse the absorbers into the polymer surface suffers from the defect that the polymer, or at least its surface region, must be heated to the melting point. This may result in an uneven or wrinkled polymer surface being formed upon cooling and solidifying of the polymer. Furthermore, the physical properties of the polymer may sometimes be deleteriously affected by this melting of the polymer. In the aggressive solvent technique an ultraviolet radiation absorbing compound is dissolved in a solvent which is aggressive towards the polymer, such as polycarbonate. Typical aggressive solvents for polycarbonate include chlorinated hydrocarbons, esters, or aromatic hydrocarbons. When these solutions are applied onto the surface of a polycarbonate article the aggressive solvent functions as a softening or swelling agent for the polymer surface allowing the absorber to diffuse into the softened or swelled polymer surface regions. However, the aggressive nature of these solvents causes problems. Surface imperfections can occur during coating and prolonged contact between the polymer and the aggressive solvent can lead to etching, hazing and crazing of the polymer. Using ultraviolet radiation stabilizing solutions containing an ultraviolet radiation absorbing compound which is more soluble in the polymer than in the stabilizing solution rather sharply limits the number and type of ultraviolet radiation absorbing compounds which may be used. Also, a large volume of the stabilizing solution must be used. This requires the use of large amounts of ultraviolet radiation absorber which is a rather expensive proposition.
Thus, there is a need for an economical and effective method for protecting polymeric, particularly polycarbonate, articles from the degradation caused by ultraviolet radiation. The instant invention provides such a method.